Mobile telecommunication networks can be divided into circuit switched networks and packet switched networks. In circuit switched networks, a circuit for communication is allocated prior to the beginning of the transmission. An example of such kind of a network is the GSM network (Global System for Mobile Communications). In this application circuit switched networks are denoted as CS networks.
In GSM a mobile services switching center an MSC is connected not only to other mobile services switching centers of the operator's network but also through a gateway mobile services switching center GMSC to other networks, such as a public telephone network PSTN, another public land mobile network PLMN or an ISDN network. Subscriber information of a mobile station MS is stored permanently in the Home Location Register HLR and temporarily in that Visitor Location Register VLR in the area of which the mobile station MS is located at each time. Thus, for each mobile terminal the home location register HLR stores the MSISDN number of the terminal, IMSI (International Mobile Subscriber Identity), and routing information to that visitor location register VLR that knows location of the mobile terminal at that moment. In other words, mobile station's location information stored in the registers of the mobile communications system is used in routing of calls. Based on location information available from the home location register HLR and from the visitor location register VLR, the incoming call can be routed to that mobile services switching center MSC in the area of which the mobile terminal of the called subscriber is located. Normally the call is routed to a GMSC in the called subscriber's home network. The GMSC sends, using the MAP protocol, an inquiry to the HLR. The HLR asks visitor location register VLR under which the called subscriber is currently located for information on the called subscriber for setting up the call. The visitor location register VLR gives the required information about the mobile station MS, including routing information identifying MSC where the called subscriber is located. That routing information is called Mobile Station Roaming Number MSRN.
The HLR responds with a message containing the MRSN to the gateway mobile services switching center GMSC. The GMSC sends an initial address message IAM towards the MSC, identified by the MSRN, under which the called subscriber is currently located.
It is common to all circuit switched networks that they use E164 numbering system, i.e. MSISDN numbers are shared between operators, each operator holding its own range of MSISDN numbers.
A new packet switched network enabling operators of mobile telecommunications networks to offer their subscribers multimedia services based on and built upon Internet applications, services and protocols is being standardized in 3GPP (3rd Generation Partnership Project). This network is called IP Multimedia Subsystem (IMS). The more detailed implementation of IMS network is beyond the scope of this application and is therefore not discussed in detail. Reference is given to the 3GPP specification documents for a reader is interested to have more information about this topic.
I general IMS should enable the convergence of, and access to, voice, video, messaging, data and web-based technologies for the wireless user, and combine the growth of the Internet with the growth in mobile communications. IMS is a packet switched system where in addition to E.164 number mobile terminals can have a logical name (e.g. SIP URL as defined in RFC2543 and RFC2396).
The core network part of IMS network is called IP Multimedia CN subsystem (IM CN SS) and it comprises all core network elements for provision of multimedia services. This includes the collection of signaling and bearer related network elements. The element of IMS network that is most relevant to this invention is Media Gateway Control function (MGCF). Media Gateway Control function is responsible for the inter-working with circuit switched networks such as PSTN. Again the actual implementation of MGCF is irrelevant. In the viewpoint the invention presented here it is sufficient that the packet switched network has an entity or entities that are capable of receiving signaling form and sending signaling to circuit switched network.
IP multimedia services are based on an IETF defined session control capability that, along with multimedia bearers, utilizes the packet switched domain. In order to achieve access independence and to maintain interoperation with fixed terminals across the Internet, the IP Multimedia Subsystem attempts to be conformant to IETF “Internet standards.” Therefore the interfaces conform to Internets standards when an IETF protocol has been selected. The most used protocol is SIP (Session Initiation Protocol).
Usually different MSISDN number ranges are allocated to the different operators. For example the subscribers in the operator A's network have they MSISDN number in a different range that subscribers from the network of operator B. However, arrangements have been made for offering subscribers having an MSIDN number to port the number to another networks. Mobile Number Portability (MNP) is the ability for a UMTS or GSM mobile subscriber to change the subscription network within a portability domain whilst retaining the original MSISDN.
In order to enable the mobile number portability it has been developed a function called Signaling Relay Function for support of MNP (abbreviation is MNP-SRF). In a PLMN that supports mobile number portability, messages sent to an HLR may be relayed by said function MNP-SRF. Depending on the implemented solution, on the type of message (call-related or non-call-related) and on the porting status of the called subscriber, the MNP-SRF may modify the called party address and route the message to a different HLR or to the subscription network or respond with routing information identifying the called subscriber's subscription network.
FIG. 1 illustrates the role of signaling relay function MNP-SRF for support of MNP. A user of mobile terminal MSA in CS network A wants to call subscriber B whose MSISDN number is held by an operator of CS network B but who is at the moment subscribed to network C. Mobile terminal MSA originates a call to the recipients MSISDN. Mobile switching center sends an IAM message containing the recipients MSISDNB number to gateway MSCA of the CS network A. The MSISDN number tells the number range holder network of MSB wherein GMSCA sends a message to GMSCB in the recipient's number range holder network B, phase 1. GMSCB requests routing information by sending a SRI message (Send Routing Info) to MNP-SRFB, phase 2. The SRI message contains the recipients MSISDN-number. When MNP-SRFB has received the message, it analyses it, makes a database inquiry and identifies the MSISDN being ported into network C. MNP-SRFB has in the database knowledge about all its MSISDN numbers that have ported to other networks and also routing information to said networks.
MNP-SRFB returns to GMSCB a message including routing information towards network C and MSISDNB, phase 3. Now gateway mobile switching center B sends IAM message including routing info and the MSISDN number to GMSCC of network C, phase 4. The GMSCC requests routing information by sending a SRI message to the MNP-SRFC, phase 5. The SRI message contains the recipient's MSISDN number, the routing information received from the GMSCB is not provided to the MNP-SRF. When MNP-SRFC has received the message, it analyses it, makes a database inquiry and identifies the MSISDN being ported into network C. MNP-SRFC has in the database knowledge about all other network's MSISDN numbers that have ported to network C and also routing information towards the correct HLR in network C. The MNP-SRFC modifies the called party address of the SRI to point towards the correct HLR and routes the message towards the HLR, phase 6. The HLR requests MSRN from the VLR and responds with a message containing the MSRN to the GMSC, phase 7. After that the call is routed through MSC to mobile terminal MSB.
In the example above all the networks are circuit switched networks (CS). The situation is, however, different when the PS and CS domains coexist and there are subscribers having access to services of both domains. If a subscriber has subscriptions in both CS and PS domains then it is possible than the same E.164 number can be used for PS sessions like IP multimedia session and CS speech telephony. This allows subscribers who originally had an E.164 MSISDN number to retain the same number for receiving communications in the IMS domain and also in the CS domain when outside IM coverage.
If the subscriber has chosen to keep his CS MSISDN number even when he has a new PS subscription then the call directed to this number are directed to the CS domain. This is due that the MSISDN number is at the range of the CS operator. So when a person is called to this number a CS originated call will be normally routed to the home location register HLR of the CS home network in order to reach the mobile subscriber. This can be for example due there is no MS roaming number (MSRN), like in the IMS network. MSRN is used in the GSM for routing the call to the roaming subscriber in the visited MSC (VMSC).
This kind of routing is not a problem if the subscriber prefers CS domain to PS domain. It is however quite probable that when packet switched networks start rolling out that subscribers start to prefer those. Then the above mentioned prior art type routing because the basic assumption is wrong. If the subscriber spends say 90% of his time using PS connection then 90% of the calls directed to him are first tried to be routed via the HLR. This is clearly not an optimum way of using signaling resources.